Here’s a list of Mathematical Applets that are useful:

Matrix Calculator:
You can calculate your matrix here. Very useful in computing eigenvalues, eigenvectors, characteristic polynomials and other matrix expressions.

Intergrator
Here’s useful tool to do your intergration. Just type in anything and it will try its best to integrate.

Differential Equations
Useful webpage for those who wish to understand Differential Equations.

Fourier Series
A good website to learn more about Fourier Series.

3D graph plotting
A nice applet for you to understand how 3D graph looks like.

Tracés animés
Plot 2D and 3D surfaces.

Vector Field Analyser
Help you to visualise vector field.

Here’s another list of applets/softwares:

Astronomical Software
Stellarium
Stellarium is a free open source planetarium for your computer. It shows a realistic sky in 3D, just like what you see with the naked eye, binoculars or a telescope. It is being used in planetarium projectors. And I used this for one of my modules- Heavenly Mathematics. (This link from my professor, Prof Helmer Aslaksen)

Celestia
This is another free software for those who are interested in heavenly objects. This free space simulation that lets you explore our universe in three dimensions. But I am not really used to Celestia. I prefer Stellarium.

Hope you may find it useful. (All the Mathematical applets links are from my professors- Prof Victor Tan and Prof Brett McInnes.)